The constant-current generating circuits commonly in use are designed to stabilize the current value against variations in the source voltage, and undergo thermal compensation so as to keep the constant current value regardless of changes in the ambient temperatures. The known circuits utilize an extrapolated voltage value of an energy band gap in silicon, thereby ensuring that the circuits supply constant voltage or current independently of temperatures. To this end, however, a current must be produced which is dependent on temperature coefficients of the voltage between the base and emitter (hereinafter referred to as "base-emitter voltage") of a transistor.
To produce such a current, under the conventional practice a circuit shown in FIG. 1 is employed. The circuit is designed to stabilize the current independently of variations in the source voltages, and to produce a current dependent on the base-emitter voltage of a transistor against the ambient temperatures. In other words, the circuit is designed to produce a current having a negative temperature coefficient dependent on the temperature coefficient of the base-emitter voltage "VBE" of a transistor.
In FIG. 1 a first transistor Q1 and a second transistor Q2 are NPN types, whereas a third and a fourth transistors Q3 and Q4 are PNP types. The base of the first transistor Q1 and the emitter of the second transistor Q2 are connected to one of the terminals of a first resistance R1, and the collector of the first transistor Q1 is connected to the base of the second transistor Q2 and one of the terminals of a resistance R0. The collector of the second transistor Q2 is connected to the collector and base of the third transistor Q3, and to the base of a fifth transistor Q5. The emitter of the first transistor Q1 is connected to the other terminal of the first resistance R1, and the junction is connected to an earthing terminal GND which is a first potential point. The other terminal of the resistance R0 is connected to the emitters of the third transistor Q3 and of the fifth transistor Q5, the junction of which is connected to a source terminal Vcc which is a second potential point. A power supply is provided between the earthing terminal GND and the source terminal Vcc, so as to operate the circuit. The collector of the fifth transistor Q5 is connected to an output terminal "OUTPUT", and a load L is connected between the "OUTPUT" and the earthing terminal GND. A current is supplied to the load L.
The circuit is operated as follows:
When the power is turned on, a current flows through the base of the second transistor Q2 via the resistance R0, and passes through the emitter thereof. The current flows through the first resistance R1 and the base of the first transistor Q1, and eventually reaches the earthing terminal GND. In this way the circuit starts to operate. As a result a negative feedback is effected by the first and the second transistor Q1 and Q2, and the first resistance R1. Thus the divident of the base-emitter voltage VBE (Q1) of the first transistor Q1 by the resistance value of the first resistance R1 is obtained as the collector current of the second transistor Q2. EQU Ic(Qpl 2)=VBE, (Q1)/R1 (1)
wherein the Ic (Q2) represents the collector current of the second transistor Q2 whereas the base current of each transistor is ignored on assumption that the d.c. current amplification factor hFE of the first, second, third and fifth transistor Q1, Q2, Q3 and Q5 is fully high.
The collector current of the second transistor Q2 is supplied to a current mirror circuit constituted by the third and the fifth transistor Q3, Q5, thereby obtaining the collector current Ic (Q5) at the OUTPUT, the characteristic of which current is decided by the base-emitter voltage of the first transistor Q1. In other words, if the base-emitter junction area of the third transistor Q3 is made equal to that of the fifth transistor Q5, the collector current of the third transistor Q3 becomes equal to that of the fifth transistor Q5. EQU Ic(Q5)=Ic Q2) (2) EQU Ic(Q5)=VBE(Q1)/R1 (3)
The conventional constant-current generating circuit is constituted in the aforementioned manner. The collector current of the first transistor Q1 is decided by the sum of the base-emitter voltages of the first transistor and of the second transistor. Under this system a voltage applied across the both terminals of the resistance R0 is likely to vary dependently on the variations in the source voltage. Consequently, the current flowing through the resistance R0 varies, which causes the collector current of the first transistor to change. As a result, the base-emitter voltage of the first transistor varies. Finally, the current flowing through the first resistance and the load L is likely to change dependently on variations in the source voltage. This is a great disadvantage of the conventional constant-current generating circuits.